To protect aircraft from infrared radiation (IR) seeking missiles, it is necessary to shield those portions of the aircraft's engines which get hot enough to emit infrared radiation. It has been determined that the most effective type of IR suppressor is of the sandwich type construction consisting of a hot surface, a cooling core and a cold surface. Since the IR suppressor has to effectively shield compound structures without significantly increasing the weight of the aircraft, it is extremely important that the suppressor be of light weight and capable of being produced in compound curvature shapes.
Over the last few years a number of major advancements have been made in the construction of light weight compound curvature suppressors; however, the advanced construction techniques are prohibitively expensive and the suppressors constructed thereby are usually structurally unreliable and not thermally accommodating. The most common technique currently being utilized is to machine or fabricate, utilizing highly sophisticated machines and expensive dies, complementary hot and cold plates or sheets of identical shape with one of the plates having reduced dimensions, thereby allowing the smaller plate to serve as the inner face of a compound curvature sandwich.
The complementary plates must match perfectly in order to enable a structurally sound bond to be achieved between the plates and the core which is usually of a honeycomb or corrugated construction. After the precisely matched plates have been formed the core must be machined to fit between the plates. As is well known, honeycomb or corrugated core is extremely difficult to machine to precise tolerances and this is what causes most of the problems. Since honeycomb or corrugated cores are somewhat flexible, they can be deformed to abut the inner surface of the outer plate or sheet to which it is bonded; however, due to the tolerance variations in the core when the inner plate is placed in position only a portion of the core will come into contact with the rigid inner plate, thereby preventing a complete structurally sound bond between the core and the inner plate. Since both the inner and outer plates are rigid there is no economically feasible method of obtaining a structurally sound bond between the inner and outer plates and the core and, therefore, the rejection rate of compound curvature infrared suppressors is extremely high.
An additional problem which is particularly acute in an aircraft environment is the differential temperature on the hot and cold, inner and outer plates of the suppressor; for example, the hot inner side of the suppressor may be exposed to temperatures as high as 1800.degree. F. while the cold outer side may be exposed to ambient or external temperatures as low as minus 120.degree. F. This thermal gradient creates great stress within the structure, thereby producing fractures or weld breaks and braze separations between the core and the face plates, while a small degree of braze separations would normally be acceptable, when combined with a structurally poor initial bond it may prove disastrous.
In addition to compound curvature infrared suppressors, there is also a need for compound curvature sound suppressors. Over the last few years the general public and various government agencies have become extremely interested in reducing the amount of noise emanating from various types of power plants but particularly from aircraft engines. Mufflers have been developed which effectively reduce the noise of automobile engines but such mufflers are not suitable for aircraft engines. The same construction techniques that are utilized to produce compound sandwich structure for infrared suppressors can also be used for sound suppressors since the same problems need to be overcome both with regard to compound shapes and thermal gradients. Additionally, sound suppressors can be structurally damaged by vibration due to sound which, in conjunction with the damage caused by the thermal gradient causes the structure to have a very limited useful life.